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Hi
sorry about the cherry barb. It's very difficult to see what's going on from the photo. could be a number of things that cause the swelling. It could be bloat, could be constipation, could be an injury or it could be a parasite about to break out of the skin

You might want to google any one of those and if you come up with something you think fits, come back and let us know. We'll see if we can help..
In the mean time, how's your water? Ammonia? Nitrites? Nitrates?
If you've got anything but 0 ammonia and nitrites, or if your nitrates are about 20 or 30, you might want to make a water change. Clean water can sometimes be very beneficial.